Solenoids used in many automobiles, trucks and other industrial machinery are subjected to adverse environmental conditions which would severely affect both the action of the solenoid and its operating life if such solenoids were left unprotected. This problem is present not only in motor vehicles and industrial machinery utilizing solenoids, but also in devices where intricate parts of movable members are subject to adverse environmerital conditions.
A known arrangement for the protection of extensible members provides a corrugated sleeve or boot which has one open end sealed to the reciprocating shaft and a second open end sealed to the housing which receives and drives the shaft. This boot is permitted to act as an accordion, retracting and extending along with the reciprocating shaft. This arrangement however, is not adequate in protecting solenoids or other intricate parts in that the volume enclosed within the sleeve is constantly varying and subsequently so is the pressure being exerted on the solenoid. As the shaft, and consequently the sleeve, are retracted, the volume within the sleeve substantially decreases while the pressure within the sealed boot is necessarily substantially increased. This increase in pressure within the boot adversely affects the action of the solenoid and subsequently the movement of the shaft, while expending some of the fluid, be it air, oil or similar fluid, from the boot. Likewise, when the shaft is returned to its original extended position the volume is increased and the pressure decreased to a point where a vacuum occurs within the boot and due to such a vacuum, the movement of the shaft is restricted and contaminated air is drawn into the boot. While a protective boot of this type provides protection from direct exposure to the environment, the action of the solenoid may be affected due to the pressure changes, and some contaminants nevertheless will enter the boot, both of which may ultimately damage the solenoid or similar device within the boot.
One solution to the formation of excessively high pressures resulting in the subsequent existence of a vacuum was to provide the boot with one or a plurality of small holes which would allow the air or other fluid to readily escape and reenter the boot. However, is solution is impracticable where a contaminant free environment is critical to the operation and continued life of the device being protected.
Other arrangements have been developed for the protection of solenoids and similar devices including that set forth in U.S. Pat. No. 3,093,633 to Delaporte et al which discloses the use of a diaphragm for protecting a reciprocating shaft from adverse environmental affects. A diaphragm having one end sealed to the housing of the electromagnetic device and the other end sealed to the shaft is employed, and as the shaft reciprocates, the diaphragm turns inside out. From this construction, a pressure change must occur immediately before and after the diaphragm reverses it direction of curvature. Also, because the diaphragm is subjected to a complete 180.degree. reversal of its position during each stroke, the diaphragm may become fatigued and fail prematurely.
U.S. Pat. No. 2,858,487 and 2,853,660 to Immel disclose the use of a dual diaphragm arrangement to protect a solenoid assembly. The two diaphragms are positioned on and sealed to opposite ends of the solenoid assembly with the concave portion of one diaphragm facing the convex portion of the other. Therefore, when the shaft reciprocates the movement of the first diaphragm is matched by that of the second thereby maintaining a constant volume within the housing. However, this type of arrangement may be utilized only in conjunction with those assemblies in which the reciprocating shaft is accessible from both sides of the assembly which in turn requires a special housing for its use.
Other attempts have been made to protect solenoids and similar assemblies having axially reciprocal shafts from adverse environmental effects such as providing gasket seals as shown in U.S. Pat. No. 1,978,916 to Stolp et al or by additional diaphragm arrangements as shown in U.S. Pat. No. 2,427,630 to Snyder and U.S. Pat. No. 3,142,790 to Baier et al. Each of these solutions encounter obstacles similar to that of the boot when applied to assemblies where only one end of the reciprocating shaft is accessible. These arrangements result in a substantial change in the volume within the housing and may result in the above-mentioned adverse affects.
Yet another attempt in providing a protective boot for solenoids is illustrated in U.S. Pat. No. 4,407,517 to Neyer. A protective boot of elastomeric material is sealed both to the housing and the reciprocatable shaft forming a fluid chamber. However, a boot of this type is limited in the length of stroke in which the shaft may travel and the active life of the boot is limited in that during each stroke of the reciprocating shaft, at least a portion of the boot will pivot 180.degree. about a flexible joint which may, over time, fail, and require replacement. Also, the expansion of the boot during the retraction of the shaft requires that a portion of the boot extend beyond the sealing connection between the shaft and boot. The expansion of the boot may be greatly inhibited if the shaft to be reciprocated is of a greater diameter than the sealed connection, or if the shaft is of a short length and is attached to a larger body.